Toshio Kurobori

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystals

Laser-active F2 centers were produced in lithium fluoride (LiF) at a concentration of 2×1018 cm−3 by irradiating focused femtosecond (fs) laser pulses from a mode-locked titanium sapphire laser (wavelength ∼800 nm, emission pulse duration ∼100 fs). This technique was used to write waveguides embedded in LiF crystals. A refractive index change estimated from a guide propagation method was approximately +1% at a wavelength of 633 nm. Refractive index-modulated volume-type gratings were also encoded inside LiF crystals by a single interfered fs laser pulse. The distributed feedback laser structure was fabricated using the gratings thus encoded, which exhibited a room-temperature F2-color center laser oscillation at 707 nm. This demonstrates a DFB color center laser operating at room temperature utilizing photon written, permanent Bragg gratings.

Simultaneous fabrication of laser-active colour centres and permanent microgratings in lithium fluoride by a single femtosecond pulse

We report, for the first time, simultaneous fabrication of laser-active F2 and F3+ colour centres in lithium fluoride and permanent periodic gratings with fringe spacings as fine as sub-micron size by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties (absorption and luminescence) of F2 and F3+ colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as x-rays. The present technique of simultaneously fabricating laser-active colour centres and functional fine-periodic structures in photo-insensitive transparent materials may well be a useful method for making miniaturized optical devices.

Applications of Wide-Band-Gap Materials for Optoelectronic Functional Devices Fabricated by a Pair of Interfering Femtosecond Laser Pulses

A pair of interfering near-IR femtosecond pulses from a mode-locked Ti:sapphire laser with a regenerative amplifier have been applied to wide-band-gap materials such a lithium fluoride (LiF) to create laser-active F2 and F3+ color centers and to encode nonerasable periodic gratings with fine fringe spacings of submicrometer size simultaneously. Using such a photon-written microstructure, a distributed-feedback LiF laser oscillation based on F2 color centers has been realized at room temperature. A lasing output performance with a linewidth of less than 1 nm, slope efficiency of ~10%, and beam divergence of ~20 mrad was obtained at 710 nm.

Lifetimes of some excited F-aggregate centres in LiF

Radiative lifetime measurements of some excited F-aggregate centres (R1, R2, F2, F3+, N2 and F2+) in LiF have been made. The typical lifetime values of the near-IR (910 nm), red (670 nm), green (530 nm) and blue (480 nm) emission bands obtained by the excitation into their absorption bands are about 30.0, 18.5, 8.5 and 6.5 ns, respectively. These measured lifetimes except for the F2+ centre appear to be temperature independent for temperatures ranging from 77 to 300 K. Estimates of the radiative lifetimes, the gain cross sections and the luminescence oscillator strengths for the F2, F3+ and F2+ centres are also given.

Development of accurate two-dimensional dose-imaging detectors using atomic-scale color centers in Ag-activated phosphate glass and LiF thin films

Novel disk-type X-ray two-dimensional (2D) imaging detectors have been developed by utilising atomic-scale defects as minimum luminescent units such as radiation-induced Ag-related species in Ag-activated phosphate glass and F-aggregated centres in lithium fluoride (LiF) thin films. Such luminescent detectors are based on the radiophotoluminescence (RPL) and photoluminescence (PL) phenomena, respectively. Accurate accumulated 2D dose distributions with a high spatial resolution of micron order over large areas and a wide dynamic range covering 8 orders of magnitude were rapidly reconstructed and were successfully demonstrated for the first time by combining the Ag-doped glass with LiF thin films. These detectors should be suitable for X- and gamma-ray imaging in radiation diagnostics and clinical radiotherapy.

Cubic and tetragonal Ce3+ ions in strontium fluoride

Optical absorption spectra in the vacuum ultraviolet region and high resolution emission and absorption spectra in the ultraviolet region for cubic and tetragonal Ce3+ centres in SrF2 crystals were measured. Cubic centres are characterized by a 33 250 cm−1 zero-phonon line and a sharp phonon line spaced from the zero-phonon line by 410 cm−1. The decay times of cubic and tetragonal centre luminescences are 32 ns. Comparison of cubic and tetragonal spectra allows us to establish the influence of charge compensating interstitial fluorine ions on f and d levels of Ce3+ ions. Non-empirical Hartree–Fock calculations for Ce states in molecular clusters of alkaline-earth fluorides were performed. To make the calculated transitions coincide with the experimental ones one needs to take into account a shift of Ce3+ and interstitial F− towards each other by 10% of the initial distance.

Room temperature visible distributed-feedback colour centre laser

The distributed-feedback (DFB) colour centre laser is described. The DFB laser action at 680 nm is observed by pumping on LiF crystal containing F2 centres with the fringes formed by the interference of two coherent light beams.

Distributed-Feedback Color Center Lasers Using N2 Centers in KCl

We report on a distributed-feedback (DFB) color center laser based on N2 centers in KCl. A periodic grating with fringe spacings as fine as 440 nm was produced by means of a two-photon coloration process using two interfering beams. Coloration and photoaggregation processes and the DFB laser action can be availed by the fourth- and second-harmonic and the fundamental lines of a Q-switched Nd:YAG laser, respectively. The linewidth of the DFB output is less than 0.3 nm in the spectrally important 1300 nm region.

Lifetime of F2 and F+3 centers in LiF crystals doped with magnesium

Abstract The influence of Mg impurity in an LiF crystal on the lifetime of the relaxed excited state of the F 2 and F + 3 centers is investigated for the first time by using the time-correlated photon counting technique at room temperature (RT). It is found that the radiative lifetime of the excited F 2 centers is almost independent of the Mg impurity concentration, while that of the excited F + 3 centers varies strongly with the corresponding impurity concentration.

Optical Properties of Photostimulable Near IR Centers in BaFCl Single Crystals Produced by Two-Photon Excitation

The stable and photostimulable F-aggregate centers, besides the already established isolated centers, located in the near IR region in BaFCl single crystals are reported. These F-aggregate centers are producible by two-photon excitation and the production rate can be enhanced by monovalent doping like incorporating K+ in BaFCl. The F-aggregate centers in BaFCl have been characterised by their growth behaviour, room temperature stability, optical bleaching, emission and the temperature dependence of the PSL lifetimes. It is a first time demonstration as applied to BaFCl single crystals to produce F-aggregate centers more efficiently by using the fourth harmonic of a Q-switched Nd:YAG laser as compared to X-irradiation.